1. Field of the Invention
The present invention relates to apparatus for connection to a communications line and more particularly, but not exclusively, to power supply circuits for use with line powered telecommunications apparatus. The invention also relates to interrupt monitoring circuits included in such apparatus.
2. Related Art
Telephones, particularly those with features such as amplification, are known to draw line current for powering from the telephone line. These circuits are only active when a telephone is in the "off-hook" condition and do not usually draw current when not in use.
Apparatus, such as facsimile transmission and receiving apparatus or telephone answering machines, which include monitoring circuits normally require relatively high power and include battery or mains electricity power supply sources.
As may be appreciated the `A` and `B` wires of a telephone line in the quiescent state have a voltage difference of approximately fifty volts D.C. in United Kingdom telephone exchanges. This voltage is provided so that a telephone line circuit in the exchange can detect looping of the line by a customer instrument by virtue of the current drawn. Such detection will occur if, say, a current of twenty milliamps or greater is drawn, but some circuits, particularly for longer lines, may be made more sensitive.
Thus if a telephone company wishes to provide line-powered apparatus, for example for telemetry applications, the current drawn from the telephone line must be less than the loop detection at lower current limit. Further, since energy consumption is not without cost, if every telephone line draws current in the quiescent state, such current draw must be minimized.